The invention described herein was made by employees of the U.S. Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
1. Field of the Invention
The present invention generally relates to ultrasonic measurement techniques for non-destructive evaluation (NDE) of materials and structural members, including the determination of fatigue damage. More particularly, the present invention relates to improved ultrasonic-based approaches to determining the nonlinearity parameter (xcex2) for assessing the fatigue damage to structural members.
The fields of use include, inter alia, ultrasonic fatigue sensor technology, process monitor technology for optimum strength of materials, bond strength analysis technology, and other nonlinear acoustic data information technology.
2. Background
NDE of materials involves the inspection of materials without having to damage the materials or dismantle structures to which the materials are incorporated. Among the many important NDE operations are the inspection of aircraft, bridge, and building structural members to detect fatigue damage that could possibly lead to catastrophic failure, as well examining bonds (e.g., adhesive junctions) between members for indications of prospective failure.
Prior art methods of fatigue damage detection include bombarding material under test with acoustic finite amplitude waves in the ultrasonic range, and examining, via a transducer, the response waves (including the fundamental and harmonics) produced by the material to determine a nonlinearity parameter (xcex2) which may be correlated to material fatigue.
One prior art approach of note requires that a piezoelectric transducer be bonded directly to the material being tested. The test material is excited by introducing finite-amplitude ultrasonic waves perpendicular to the surface of the test material, whereupon the surface vibration energy is received by the transducer and analyzed to determine the nonlinearity parameter (xcex2). Among the drawbacks of this approach is the fact that the results must be corrected for the layer of bond material between the test material and the transducer, and it is assumed that the test,material and transducers are both flat and parallel. Further, the attenuation characteristics of the test material must also be known. This approach is therefore impractical at times, and can lead to over-correction (assuming that corrections can be practically made), and therefore inconsistent results.
Another approach of note uses an air-gap capacitive transducer. That is, the surface of the test material serves as part of an air-gap capacitive transducer, with the surface being xe2x80x9cfree.xe2x80x9d The direction of acoustic wave propagation is ideally perpendicular to the test material surface, although corrections can be made if the relative wave propagation angle is known. While this approach generally works well if the parameters are carefully controlled, as in a laboratory, it does not work reliably, for example, if the test material surface is not optically flat. Therefore, it is not a practical approach for most field testing of structures.
Yet another approach of note known as laser interferometry impinges laser light on the surface of a vibrating test material. The reflected light is analyzed, employing for example, Michelson or Fabry-Perot demodulation techniques, by complex and often expensive equipment. While a strong approach, it requires that the test material have good optical reflection characteristics, which is not the case for many in-field structures. Additionally, the equipment must be optically aligned (due to temperature and other environmental changes) often to produce reliable results. Those skilled in the art will also appreciate that the excitation waves must be aimed so that the reflection energy exits the surface vibrations precisely in the center of the sound field, which is extremely difficult to locate in practice.
What is therefore needed but sorely lacking in the prior art, is an ultrasonic transducer method of NDE that is reliable in the field, does not require a highly optically reflective test material surface, is cost-effective, and requires minimal preparation of the test equipment and test material in the field.
In view of the aforementioned problems and deficiencies of the prior art, the present invention provides a system for measuring acoustic nonlinearity (xcex2) in test materials. In at least one embodiment, the system can at least include an acoustic signal generator adapted to apply an acoustic signal to a test material, a dielectric electrostatic ultrasonic transducer (DEUT) comprising a plate member and a dielectric member coupled to the plate member, the dielectric member being a high dielectric constant insulator, and being adapted to convert received ultrasonic energy into an output electrical signal, and a measurement system coupled to the DEUT, the measurement system being adapted to calculate the nonlinearity xcex2 in response to the output signal from the DEUT. The DEUT is adapted to be loosely mounted to a surface of the test material, and the DEUT is adapted to receive acoustic energy from the surface of the test material.
The present invention also provides a method of measuring acoustic nonlinearity (xcex2) in test materials. The method can at least include the steps of generating and applying an acoustic signal to a test material, providing a DEUT comprising a plate member and a dielectric member coupled to the plate member, the dielectric member being a high dielectric constant insulator, and being adapted to convert received ultrasonic energy into an output electrical signal, and loosely mounting the DEUT to a surface of the test material. The method further can at least include the steps of via the DEUT, receiving acoustic energy from the surface of the test material, and via a measurement system coupled to the DEUT, calculating the nonlinearity xcex2 in response to the output signal from the DEUT.